Most of the known SySt have been discovered during photographic
objective-prism surveys of the Galactic plane and Bulge (e.g.
[1]}, [2]}, [3]}), many
of which were initially classified and studied as planetary nebulae. Their
true nature was generally unveiled by infrared observations catching the
presence of a cool giant in the system
[4]}, [5]}, [6]}. The
same objective-prism technique was applied also to the Magellanic Clouds and
several SySt were discovered there too
[7]}, [8]}. Searches for SySt in
external galaxies has steadily progressed over the years
[9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, and the
current total of known extra-galactic SySt amounts to \(\sim \) 70 systems
[18]}, [19]}, [20]}. New
surveys aiming to SySt within the Local Group [21]}
promise to increase such numbers, especially for the burn-SySt type.
